In recent years, digitization of analog circuits has progressed as semiconductor processes have been shrunk, and efforts for reduction in circuit scale, improvement in circuit stability, reduction in power consumption and the like have been made. Further, support for process shrinkage is facilitated by digitization, leading to an advantage of improvement in development efficiency.
Also in the field of signal processing for optical discs, it is possible to change a signal processing circuit from an analog signal processing circuit to a digital signal processing circuit by using a technique such as PRML (Partial Response Maximum Likelihood), whereby reproduction performance is improved.
Accordingly, the conventional signal processing apparatus is required to efficiently perform analog-to-digital conversion (hereinafter referred to as A/D conversion) at a part that performs A/D conversion, that is, it is required to convert an analog signal into a digital signal at a maximum amplitude within a dynamic range of an A/D converter.
FIG. 20 is a block diagram illustrating a conventional signal processing apparatus of this type.
The conventional signal processing apparatus comprises a high-pass filter 101 which cuts off low-frequency bands of an input reproduction signal to remove a DC offset; a variable gain amplifier 1 which gives a gain based on an inputted gain control signal to the inputted reproduction signal from which the DC offset is removed by the high-pass filter 101; an A/D converter 3 which receives the output signal from the variable gain amplifier 1, and subjects the signal to A/D conversion; a peak detector 4 which performs peak detection from sampling data obtained by the A/D converter 3; a bottom detector 5 which performs bottom detection from the sampling data; an amplitude detector 6 which calculates a signal amplitude at the input of the A/D converter on the basis of the detected peak value and bottom value; and a gain controller 8 which controls the variable gain amplifier 1 so as to make the A/D input amplitude constant, on the basis of the amplitude information obtained by the amplitude detector 6.
In the above-mentioned construction, the signal amplitude at the input of the A/D converter 3 is detected, and feedback control is carried out to make the signal amplitude constant, whereby the input signal to the A/D converter 3 can be effectively nestled in the dynamic range of the A/D converter 3.
By the way, there is a DC free method as a recording modulation method for an information recording medium such as an optical disc. In this method, recording is carried out so that the ratio of “H” period to “L” period of a signal becomes 50:50.
However, when the recording modulation method for the information recording medium is not DC free or when recording marks are formed in lengths longer or shorter than original lengths due to variations in the manufacturing stage of the information recording medium although the DC free method is employed, the ratio of “H” period to “L” period of the signal deviates from 50:50, resulting in a phenomenon that the average DC level of the reproduction signal deviates from the center position between the upper and lower peaks of the reproduction signal. This phenomenon frequently occurs when the recording condition is not optimized, and it is generally called “asymmetry”. When a reproduction signal having asymmetry passes through the high-pass filter 101, the DC component is cut off, and a DC offset is generated in the input signal of the A/D converter 3 as shown in FIG. 21. As the DC offset increases, the reproduction signal waveform undesirably exceeds the input dynamic range of the A/D converter 3, and a portion of the waveform is lost, whereby correct A/D conversion cannot be carried out.
However, this problem can be solved by a second prior art that is disclosed in, for example, Patent Document 1 to be described later. FIG. 22 is a block diagram illustrating a signal processing apparatus disclosed in Patent Document 1, which is appropriately rewritten so as to facilitate correlation with the block construction shown in FIG. 21.
The signal processing apparatus according to the second prior art comprises a variable gain-amplifier 1 which gives a gain based on an inputted gain control signal to an inputted reproduction signal; an offset unit 2 which gives a DC offset based on an inputted offset control signal to the output of the variable gain amplifier 1; an A/D converter 3 which performs A/D conversion for the output of the offset unit 2; a peak detector 4 which performs peak detection from sampling data obtained by the A/D converter 3; a bottom detector 5 which performs bottom detection from the sampling data; an amplitude detector 6 which calculates a signal amplitude at the input of the A/D converter 3 on the basis of the detected peak value and bottom value; an offset detector 7 which calculates a center value of the input to the A/D converter on the basis of the average of the detected peak value and the detected bottom value; a gain controller 8 which controls the variable gain amplifier 1 so as to make the A/D input amplitude constant, on the basis of the amplitude information obtained by the amplitude detector 6; and an offset controller 9 which performs control so that the center of the A/D input signal matches the center of the input dynamic range of the A/D converter 3, on the basis of the offset information obtained by the offset detector 7.
In this prior art, there are provided two control loops, i.e., a control loop for detecting the signal amplitude at the input of the A/D converter 3 to make the signal amplitude constant, and a control loop for detecting the center value of the input signal to perform offset control, whereby the signal amplitude and offset of the input signal to the A/D converter 3 are controlled so that the input signal can be efficiently nestled in the input dynamic range of the A/D converter.
Patent Document 1: Japanese Patent No. 3067298 (Pages 7–11, FIGS. 2 and 4)
However, in the case where the input signal drops as shown in FIG. 23(a) due to an adherent such as dust or a flaw on the information recording medium, because both of the gain and the offset are controlled in the second prior art described above, the gain of the variable gain amplifier 1 increases at the signal dropout portion, and the offset unit 2 is operated to return the signal back to the center, resulting in a signal having a waveform as shown in FIG. 23(b) at the input of the A/D converter 3. As seen from FIG. 23, after the reproduction signal has passed through the signal dropout portion, there occurs a wasted time from when the reproduction signal exceeds the input dynamic range of the A/D converter 3 to when the reproduction signal is pulled into the normal state where it is nestled in the input dynamic range, and data reproduction cannot be normally carried out during this time period. For example, when there is a dropout of 2 mm, a waveform abnormality continues actually for a time period equivalent to 3 mm due to the wasted time, leading to a reduction in reproduction performance.
Furthermore, since the bottom of the reproduction signal gradually approaches the center of the input dynamic range of the A/D converter 3 by offset control during the dropout period, when a result of comparison between the output of the peak detector 4 and a predetermined threshold is adopted as a dropout detection signal, the dropout detection signal is interrupted at some midpoint in the signal dropout period as shown in FIG. 23(c), whereby holding of a subsequent PLL (Phase Locked Loop) circuit or a peripheral circuit for such as focus control or tracking control cannot be correctly carried out.
Furthermore, in the above-described prior art, both of the gain and the offset must be controlled, resulting in an increase in circuit scale as well as an increase in power consumption.
The present invention is made to solve the above-described problems and has for its object to provide a signal processing apparatus and a signal processing method that can suppress a phenomenon in which a reproduction signal exceeds an input dynamic range of an A/D converter after passing through a signal dropout period, and that can improve precision of dropout detection.